Hitherto, such apparatus has generally been one of two types. These are single rotary brush pneumatic or hydraulic machines and triple rotary brush machines which are usually hydraulic. The disadvantages of the single brush machines are that the divers operating them have to resist the reaction torque of the brushes and are thus quickly fatigued, and that it is time-consuming to cover large areas. The triple brush machines are much larger and more powerful and usually have separate hydraulic propulsion motors and steerable wheels to render them capable of control by divers. The disadvantages, however, are that they are costly, cumbersome and only really effective on long straight runs on flat surfaces. All the curved surfaces are more efficiently done with a hand-held single brush machine. Triple brush machines are also extremely heavy out of water and need cranes and trailers to remove and transport them.